DESCRIPTION

HMAC is a MAC (message authentication code), i.e. a keyed hash
function used for message authentication, which is based on a hash
function.

HMAC() computes the message authentication code of the n bytes at
d using the hash function evp_md and the key key which is
key_len bytes long.

It places the result in md (which must have space for the output of
the hash function, which is no more than EVP_MAX_MD_SIZE bytes).
If md is NULL, the digest is placed in a static array. The size of
the output is placed in md_len, unless it is NULL.

evp_md can be EVP_sha1(), EVP_ripemd160() etc.
key and evp_md may be NULL if a key and hash function have
been set in a previous call to HMAC_Init() for that HMAC_CTX.

HMAC_CTX_init() initialises a HMAC_CTX before first use. It must be
called.

HMAC_CTX_cleanup() erases the key and other data from the HMAC_CTX
and releases any associated resources. It must be called when an
HMAC_CTX is no longer required.

HMAC_cleanup() is an alias for HMAC_CTX_cleanup() included for back
compatibility with 0.9.6b, it is deprecated.

The following functions may be used if the message is not completely
stored in memory:

HMAC_Init() initializes a HMAC_CTX structure to use the hash
function evp_md and the key key which is key_len bytes
long. It is deprecated and only included for backward compatibility
with OpenSSL 0.9.6b.

HMAC_Init_ex() initializes or reuses a HMAC_CTX structure to use
the function evp_md and key key. Either can be NULL, in which
case the existing one will be reused. HMAC_CTX_init() must have been
called before the first use of an HMAC_CTX in this
function. N.B. HMAC_Init() had this undocumented behaviour in
previous versions of OpenSSL - failure to switch to HMAC_Init_ex() in
programs that expect it will cause them to stop working.

HMAC_Update() can be called repeatedly with chunks of the message to
be authenticated (len bytes at data).

HMAC_Final() places the message authentication code in md, which
must have space for the hash function output.

RETURN VALUES

HMAC() returns a pointer to the message authentication code.

HMAC_CTX_init(), HMAC_Init_ex(), HMAC_Update(), HMAC_Final() and
HMAC_CTX_cleanup() do not return values.